We have not yet achieved a self-consistent theoretical scenario for the formation of the local disk galaxy population, a population which, in crude terms, is a mix of "bulged" and "bulgeless" disks. The origin of the bulges is vigorously debated. In a recent paper in the proceedings of the 1998 workshop on "The Formation of Bulges" (Carollo, Ferguson, & Wyse 1999), Renzini (1999) voices with emphasis the long-standing belief that bulges are nothing more or less than small elliptical galaxies. Basing his argument on the similarity between the magnesium line strength Mg2 versus absolute r magnitude Mr (and magnesium line strength Mg2 and velocity dispersion ; Jablonka, Martin, & Arimoto 1996) relations for bulges and ellipticals, he writes: "...The close similarity of the Mg2-Mr relations argues for spiral bulges and ellipticals sharing a similar star formation history and chemical enrichment. One may argue that origin and evolution have been very different, but differences in age distribution are precisely compensated by differences in the metallicity distributions. This may be difficult to disprove, and I tend to reject this alternative on aesthetic grounds. It requires an unattractive cosmic conspiracy, and I would rather leave to others the burden of defending such a scenario. In conclusion, it appears legitimate to look at bulges as ellipticals that happen to have a prominent disk around them, or to ellipticals as bulges that for some reason have missed the opportunity to acquire or maintain a prominent disk."
And yet, there is plenty of evidence from observations and numerical experiments that bulges of spiral galaxies may differ significantly from elliptical galaxies. In a pioneering paper, Kormendy (1993; see also Kormendy, Bender, & Bower 2002) reports, for some Sb bulges, V / values that are above the oblate line describing the isotropic spheroids in the V / - diagram (with V the maximum velocity, the mean velocity dispersion, and the mean ellipticity of the spheroid; Binney & Tremaine 1987), and makes the point that at least some of the dense structures that are seen inside the disks may actually themselves be disklike systems ("pseudo-bulges"). In numerical simulations, three-dimensional stellar structures result from secular evolution processes that are driven by dynamical instabilities inside the preexisting disks. The fire-hose (or buckling) instability that is seen in simulations of stellar disks can scatter the stars originally in a stellar bar above the plane of the disk, into what resembles a bulgelike structure (Raha et al. 1991). A stellar bar can also drive a high inflow rate of gas toward the center of the disk (Shlosman, Frank, & Begelman 1989); if a mass concentration of the order of ~ 1% of the total mass is accumulated in the center, this can disrupt the regular orbits supporting the bar and again scatter the stellar orbits above the plane of the disk (Pfenniger & Norman 1990; Norman, Sellwood, & Hasan 1996).
The disks of spiral galaxies also elude us. On large galactic scales, they are a rather homogeneous family, as indicated, for example, by their light profiles, which appear to be exponential over several disk scale lengths (de Jong 1995), the rather common asymptotically flat rotation curves (Persic & Salucci 1995), and the Tully-Fisher relation, which holds over a broad range of surface brightness and mass (Strauss & Willick 1995). On smaller scales, however, where they physically overlap with the bulges (and the rest of the inner structure), disks are not well understood, either observationally or theoretically. Within hierarchical formation schemes, the standard recipe to explain the formation of disks contains three key elements: (1) the angular momentum originates from cosmological torques (Hoyle 1953), (2) the gas and dark matter within virialized systems have initial angular momentum distributions that are identical (Fall & Efstathiou 1980), and (3) the gas conserves its specific angular momentum when cooling (Mestel 1963). These rules are routinely assumed in the (semi-)analytical descriptions of disk galaxies. In contrast, the highest resolution cosmological simulations that include both baryons and cold dark matter (CDM) find significant angular momentum loss for the baryons, especially in the central few kpcs of galaxies (Steinmetz & Navarro 1999). Furthermore, even when disks are assumed to form smoothly and conserving their angular momentum, the resulting disks are more centrally concentrated than single-exponential structures (Bullock et al. 2001; van den Bosch 2001; van den Bosch et al. 2002). Disks with high central densities are seen in the highest-resolution CDM simulations, in which the resulting galaxies have realistic sizes, but a region with low angular momentum and high density is always present at the center (e.g., Governato et al. 2003). It is still a matter of debate whether this is a feature of the structure formation model or is indicative of the lack of a proper treatment of physics (e.g. the effect of supernovae feedback; Springel & Hernquist 2002). Although the CDM simulations still have room for improvement, they could well be correct in their prediction that the central parts of disks might really have quite low angular momentum and high concentration as a result of formation. Although warm dark matter alleviates the angular momentum "catastrophe" (i.e., the loss of angular momentum by the baryons), the angular momentum distributions of warm dark matter halos is identical to that of CDM halos (Knebe, Islam, & Silk 2001; Bullock, Kravtsov, & Colin 2002). Therefore, these halos also predict an excess of low-angular momentum material.
Clearly the central regions of disk galaxies hold important clues to understanding fundamental issues of galaxy formation. Shaping a consistent theory of bulge and disk assembly requires a better understanding of nearby disk galaxies on the nuclear and circumnuclear scales. High-resolution studies of real and simulated disk galaxies are still in their infancy, but have made their first steps in the last few years. Recent reviews on disk galaxies and their subcomponents are presented by Wyse, Gilmore, & Franx (1997) and Carollo et al. (1999). In this review I focus on some recent developments on the central regions of nearby disk galaxies, and discuss some of the related important issues that require future attention. In order to remain faithful to the original studies, and following customary classification schemes, I will often discuss the results maintaining a distinction among systems of early, intermediate, and late types; however, it is this very distinction that I challenge in my concluding remarks.